North Atlantic Oscillation

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In a positive phase, both the low-pressure zone over Iceland and high pressure over the Azores are intensified, resulting in changes in the strength, incidence, and pathway of winter storms crossing the Atlantic. This results in mild and wet winter conditions in the eastern U.S., warm and wet winters in Europe, and cold and dry winters in northern Canada and Greenland. In a negative phase, a weak subtropical high and a weak Icelandic low results in fewer and weaker winter storms crossing on a more west-east pathway. This phase brings more cold air and snowy conditions to the U.S. east coast, moist air into the Mediterranean and cold air to northern Europe, while Greenland experiences milder winter temperatures. (Animation by Jack Cook, Woods Hole Oceanographic Institution)